Hydrocyclones are commonly used for separating suspended matter carried in a liquid into two discharge streams or “phases” of different density. In the mining industry, for example, hydrocyclones are commonly used to separate particulates which are located in a slurry into a heavier (“coarser”) solid phase and a lighter (“finer”) solid phase, for classification purposes.
During normal “stable” operation, the slurry enters through an upper inlet of a hydrocyclone separation chamber in the form of an inverted cone, with the heavier solid phase being discharged through a lower outlet and the lighter solid phase being discharged through an upper outlet. However, the stability of a hydrocyclone during such an operation can be readily disrupted, resulting in an ineffective separation process and whereby either an excess of fine particulates exit through the lower outlet or courser particulates exit through the upper outlet.
One form of unstable operation is known as “roping”, whereby the rate of solids being discharged through the lower outlet increases to a point where the flow is impaired. If corrective measures are not timely adopted, the accumulation of solids through the outlet will build up in the separation chamber, the internal air core will collapse and the lower outlet will discharge a coarse, rope-shaped flow of coarse solids. Roping may also result in a substantial part of the heavier phase being discharged through the upper outlet. A number of different operational conditions can cause roping, some of which include changes in the composition and viscosity of the slurry, increases in slurry feed speed, among others.
Another form of unstable operation is where the proportion of fine material being incorrectly discharged through the lower outlet progressively increases to an unacceptable level. This form of unstable operation can be caused, for example, as a result of changes in the composition and viscosity of the input slurry, decreases in the slurry feed speed, and so on.
Both of the unstable operating conditions described above can have serious impacts on downstream processes, often requiring additional processing (which, as will be appreciated, can greatly impact on profits) and also result in accelerated machinery wear.
Various techniques have been proposed to determine and correct unstable hydrocyclone operation. Most of these techniques take advantage of a notable characteristic of stable hydrocyclone operation; that is, that the heavier solid phase during stable operation will exhibit a constant umbrella shaped spray pattern as it exits the apex of the lower outlet.
One such technique is described in U.S. Pat. No. 5,132,024 which discloses a sensor mounted on an inner wall of the hydrocyclone apex and being arranged to contact the discharged flow during normal operation. The system outputs a warning when the sensor is unable to detect the flow (i.e. which is indicative of roping). However, it will be appreciated that this technique is only capable of detecting roping after it has already occurred, which may not provide operators with enough time to remedy the situation. Furthermore, the sensor is prone to accelerated wear due to its direct contact with the coarse slurry discharge. Another disadvantage is that the sensor is unable to detect the other mode of unstable operation mentioned above, which involves a bypass of fines through the lower outlet.
Another technique is outlined in U.S. Pat. No. 6,983,850 whereby a vibration sensor is provided on an outer wall of the hydrocylone lower outlet and arranged to detect vibrational changes in the wall that may be indicative of roping. While the vibration sensor disclosed in U.S. Pat. No. 6,983,850 is not subject to wear, and may be able to detect roping earlier than, for example, the sensor of U.S. Pat. No. 5,132,024, it also has a number of drawbacks. For example, the vibration sensor is only arranged to measure significant changes in the lower outlet geometry which occurs after the onset of roping. Furthermore, the vibration sensor readings can be contaminated by noise from surrounding equipment. The vibration sensor is also unable to detect a bypass of fines through the lower outlet.